1. Field of the Invention
The invention relates to an oil pump in which an inner rotor and an outer rotor having a trochoid tooth profile rotate to allow oil sucked into a pump chamber to be discharged out of the pump chamber through a discharge port.
2. Description of the Related Art
An oil pump in which an inner rotor and an outer rotor having a trochoid tooth profile rotate to allow oil sucked into a pump chamber through a suction port to be discharged out of the pump chamber through a discharge port has been used for hydraulic equipment and the like. In such an oil pump, when the discharge port is brought into communication with one of the inter-tooth chambers formed between tooth profile portions of the inner rotor and tooth profile portions of the outer rotor, the oil at the discharge port may flow into the inter-tooth chamber as a jet due to a difference in pressure between the inter-tooth chamber and the discharge port. This phenomenon is significant when the inter-tooth chamber comes into communication with the discharge port while the pressure in the inter-tooth chamber is not high enough due to many bubbles mixed in the oil. When the oil at the discharge port flows into the inter-tooth chamber as a jet, the bubbles mixed in the oil in the inter-tooth chamber may instantaneously collapse, leading to noise or erosion. Thus, oil pumps have been proposed which include a configuration reducing an increase in pressure when the inter-tooth chamber comes into communication with the discharge port to inhibit possible noise or erosion (see, for example, Japanese Patent Application Publication Nos. 2005-42689 (JP 2005-42689 A) and 2006-266161 (JP 2006-266161 A).
In the oil pump described in JP 2005-42689 A, a shallow groove is formed in a pump housing provided with a pump chamber (rotor chamber) that is in communication with the suction port and the discharge port such that the shallow groove is positioned on a trajectory circumference of a bottom land portion of the outer rotor. When the volume of the inter-tooth chamber decreases from the maximum value, the inter-tooth chamber comes into communication with the discharge port via the shallow groove.
In the oil pump described in JP 2006-266161 A, a shallow groove that is in communication with the discharge port is formed on a radially inner side with respect to the trajectory circle of the bottom land portion of the inner rotor. The shallow groove is in communication with the inter-tooth chamber via a side clearance between a side surface of the inner rotor and a housing.
When the oil pump is used, for example, to lubricate and operate a transmission in an automobile and oil is sucked from an oil pan in a transmission case, the sucked oil may contain a large number of bubbles because the oil is stirred as a result of the rotation of gears or the like. The oil pumps described in JP 2005-42689 A and JP 2006-266161 A may allow inhibition of the adverse effect (noise or erosion) of collapsing bubbles resulting from, for example, cavitation but may fail to produce a sufficient inhibition effect when the sucked oil contains a large number of bubbles. That is, the oil at the discharge port may flow into the inter-tooth chamber as a jet causing rapid elevation of the pressure in the inter-tooth chamber, resulting in noise or erosion.